Optical fiber communication systems are beginning to achieve great potential for the rapid transmission of vast amounts of information. In essence, an optical fiber system comprises a light source, a modulator for impressing information on the light, an optical fiber transmission line for carrying the optical signals and a receiver for detecting the signals and demodulating the information they carry. Increasingly the optical signals are wavelength division multiplexed signals (WDM signals) comprising a plurality of distinct wavelength signal channels.
Reconfigurable multi-channel filters are important components of optical communication systems. Conditions in an optical communication system can change as channels are amplified, added, dropped and rerouted among branches. Multichannel filters are useful in selectively adding or dropping channels and in compensating amplitude variation among different channels. Reconfigurability is needed to adapt to changing conditions.
One conventional multichannel filter is based on the well-known Mach-Zehnder Interferometer (MZI). An MZI comprises a pair of waveguiding arms extending between a pair of couplers. The input is on one arm; and the output, taken from the other arm, depends on the phase difference between the arriving signals. The amplitude of the output varies sinusoidally with wavelength.
To make the MZI into a reconfigurable multi-channel filter, a router is disposed in one of the arms to separate the channels among a plurality of channel arms. Each channel arm is provided with a phase shifter, and the channels are recombined at a second router. Control of the phase of each channel permits control of its amplitude.
A difficulty with this approach is that the MZI sinusoidal response acts as a narrow band filter. This has the drawback of narrowing the bandwidth of each channel. Accordingly there is a need for an improved multi-channel filter with enhanced channel bandwidth.